Small fossil treasures from the Panama Canal: Updating the Central American fossil faunal list

by Aldo Rincón

The ongoing screenwashing activities on small fossil mammals from Panama represent one of the most interesting paleontological research objectives of the PCP-PIRE. Small mammals can be useful as biostratigraphic tools, and some groups can also be reliable paleoecological markers. Although they are relatively common in a variety of ecosystems, small mammals can be 'hard to catch' in the fossil record because of their tiny size and relatively low preservation potential.

To date, there is only one genus of geomyoid rodents (pocket gophers and fossil relatives) formally described from the fossil record of Panama. These small fossils (six isolated teeth) were recovered from the Miocene Cucaracha Formation (c.a 17 Ma) and formally described by Bob H. Slaughter in 1981. The specimens were retrieved after an intense screenwashing effort in localities close to the Pedro Miguel locks.

Although Slaughter's localities have been lost as a result of the expansion of the canal, prospecting activities were far from ending. Since 2007, Gary S. Morgan, Curator of Paleontology of New Mexico Museum of Natural History and one of PCP-PIRE's principal investigators, has been washing and sorting matrix sediment from different localities along the canal with no remarkable success-until recently. Working with Utahna Denetclaw, Morgan has washed and sorted matrix samples from the Hodges Hill Microsite, one of the most prolific Cucaracha Formation fossil localities along the canal. About 750 pounds of raw sediment have recently sorted and the samples yielded incredible results. Despite the fact that the total sample is relatively small (screenwashing activities require more than one metric ton of raw sediment in some localities and lithologies), the total number of small mammal teeth recovered to date from the Hodges Microsite exceeds 20 specimens. Gary has also recently recovered other small teeth from the older Las Cascadas Formation, one of the main targets for microfauna sampling and one of the oldest fossil assemblages in Central America (c. a 20 Ma).

So far, the newly recovered microfauna represent at least 10 species of flying squirrels, chipmunks, small to large sized "pocket-gophers," pocket mice or kangaroo rats, small raccoon-like carnivores, small crocodiles, iguana-like reptiles and the only fossil record of tropical bats from Central America. Rodents are often recovered in sediments from the Cucaracha Formation while sediments from the Las Cascadas Formation have produced small carnivores, iguanids and bats. The sample of rodent teeth in particular has increased tremendously since the PCP-PIRE started in 2010 adding more small treasures to the Miocene faunal list. Hopefully, representatives of other small mammal groups such as marsupials (opossums), lipotyphlans (small insectivores), or lagomorphs (rabbits) could also be recovered from these fossiliferous microsites.